An increasing body of evidence supports the efficacy of single and, more recently, multiple micronutrient supplementation in reducing morbidity and mortality in susceptible populations. Our multiple micronutrient supplementation trial in HIV-positive Tanzanian women showed a significant reduction in pre-term birth, fetal loss, and low birth weight. In addition, reduced rates of HIV transmission and morbidity were seen in subsets of infants born to supplemented mothers. Because of the likelihood of multiple micronutrient deficiencies occurring simultaneously, and because of likely synergism among nutrient supplementation, our next priority is to evaluate the efficacy of multiple micronutrient supplementation in susceptible children. In this application, we propose to study the efficacy of zinc or multiple micronutrient supplementation in reducing the risk of infectious diseases and growth faltering among 2400 infants and young children in Tanzania. Infants (n=2400) born to HIV-negative women will be recruited and randomly assigned in a factorial design to either zinc, micronutrients (vitamins C, E, B1, B2, niacin, B6, folate, and B12) plus zinc, micronutrients without zinc, or a placebo given daily. Children will be followed at monthly clinic visits from age 6 weeks to 18 months, and home visits will occur at the midpoint between study visits. Data obtained will include socioeconomic status, anthropometric data (weight, length, head circumference, and arm anthropometries), dietary intake (including breastfeeding duration and frequency), hemoglobin, ferritin. and blood smear for malaria. The primary outcomes will be the incidence of diarrhea and respiratory tract infections. Secondary outcomes will be weight and length gain. A subset of children will be tested for blood concentrations of vitamin A, E, zinc and C-reactive protein, as well as measures of intestinal permeability. All children will receive a large periodic dose of vitamin A every 6 months as per standard of care in Tanzania. There is a longstanding history of collaboration between Tanzanian researchers and the Harvard School of Public Health, and Drs. Duggan and Fawzi have a long record of collaborative research in a variety of developing country settings. As experts in the fields of nutrition, pediatrics, epidemiology and maternal and child health, our team is committed to further evaluating the possible role of micronutrients in improving child health.